1. Field of the Invention
The present invention relates to self-hydrating membrane electrode assemblies (MEAs), including MEAs that have been magnetically modified, and to methods of manufacture of the same. The present invention further relates to fuel cells that require only a self-hydrating MEA and a source of fuel.
2. Background of the Related Art
A fuel cell is a device that converts the energy of a chemical reaction into electricity. It differs from a battery primarily in that the fuel and oxidant are stored external to the cell, which can therefore generate power only as long as the fuel and oxidant are supplied. Moreover, unlike secondary batteries, fuel cells do not undergo charge/discharge cycles.
A fuel cell produces an electromotive force by bringing the fuel and oxidant into contact with two suitable, but different, electrodes separated by an electrolyte. A fuel, such as hydrogen gas for example, is introduced at a first electrode, where it reacts electrochemically in the presence of the electrolyte to produce electrons and protons in the first electrode.
These electrons are then circulated from the first electrode to a second electrode through an electrical circuit connecting the electrodes. Protons pass through the electrolyte to the second electrode.
At the same time as the fuel is introduced to the first electrode, an oxidant, such as oxygen gas or air, is introduced to the second electrode, where it reacts electrochemically in presence of the electrolyte to consume the electrons that have circulated through the electrical circuit and the protons that have passed through the electrolyte.
The first electrode is therefore an oxidizing electrode, while the second electrode is a reducing electrode. Thus, in the case of H2/O2 and H2/air cells, the respective half-cell reactions at the two electrodes are:H2→2H++2e−; and   (1)½O2+2H++2e−→H2O.   (2)
The electrical circuit connecting the two electrodes withdraws electrical current from the cell and thus receives electrical power. The overall fuel cell reaction produces electrical energy according to the sum of the separate half-cell reactions above. In addition to electrical energy, water is formed at the cathode as a byproduct of the reaction as well as some heat energy.
For many practical applications, fuel cells are usually not operated as single units due, at least in part, to the relatively low electrical energy produced by individual cells. Rather, fuel cells may be connected in a series, stacked one on top of the other, or placed side by side.
A series of fuel cells (referred to as a “fuel cell stack”) is normally enclosed in a housing. The fuel and oxidant are directed with manifolds to the electrodes, and the required cooling (to dissipate the heat energy) may be provided by the reactants or by a cooling medium.
Also within most common fuel cell stacks are current collectors, cell-to-cell seals, insulations, piping, and/or instrumentation. The combination of the fuel cell stack, housing, and associated hardware is known as a “fuel cell module.”
Fuel cells may be classified by the type of electrolyte (e.g., liquid or solid) that they contain. Fuel cells using electrolytes such as the solid polymer membranes referred to as “proton exchange membranes” or “polymer electrolyte membranes” (PEMs) operate best when the PEM is kept moist with water (PEMs transfer protons more efficiently when wet than when dry). The PEM therefore generally requires constant humidification during operation of the fuel cell.
This humidification has been achieved by adding water to the reactant gases (e.g. hydrogen and oxygen or air) that pass by the membrane on each side of the MEA. The accessories required for humidification, however, add instrumentation and weight to the fuel cell as well as increasing mechanical complexity and reducing output due to parasitic power loss (the energy required to heat water for humidification can consume 15% or more of power output).
The PEM used in solid polymer fuel cells acts both as the electrolyte as well as a barrier that prevents the mixing of the reactant gases, a potentially disastrous situation. Examples of suitable membrane materials are the polymeric perfluorocarbon ionomers generally containing a basic unit of fluorinated carbon chain and one or more sulphonic acid groups. There may be variations in the molecular configurations and/or molecular weights of this membrane. One such membrane commonly used as a fuel cell PEM is sold by E. I. DuPont de Nemours under the trademark “NAFION” (tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer). Typically, best fuel cell performance is obtained using these membranes if the fuel cells are operated under fully hydrated, i.e. essentially water-saturated, conditions. Thus, the PEM must be continuously humidified during fuel cell operation.
There have been other attempts to provide the necessary humifidication to a fuel cell PEM, or eliminate the need therefore entirely For example, U.S. Pat. No. 5,318,863 to Dhar discloses solid polymer fuel cells which operate at near ambient temperature and pressure without humidification. One such fuel cell employs very thin electrodes having a slightly oversize solid PEM between and in contact with them. The PEM has either a low gram equivalent weight or has a higher gram equivalent weight but is very thin so as to permit proton transfer at reduced internal electrolyte resistance. This decreased internal electrolyte resistance is intended to permit operation of the fuel cell at mild conditions without humidification. The use of very thin membranes can permit easier conductivity of water due to the shorter transport path length, but such membranes do not exhibit long term stability and frequently permit H2 crossover (which bleeds power). U.S. Pat. No. 5,242,764, also to Dhar, discloses a fuel cell which employs a solid PEM having a central hole between and in contact with the electrodes.
All of the above described fuel cells and MEAs, however, suffer from one or more problems and/or disadvantages that limit their applicability and/or commercial potential. Most state-of-the-art fuel cells use MEAs that typically require substantial hardware to sustain fuel cell operation. This includes hardware to pressurize, humidify and/or heat the fuel cell. This hardware introduces moving parts that substantially complicate the system and dramatically increase the weight and cost, as well as increasing mechanical noise, thermal signature and complexity.
Accordingly, there remains a need in the art for a fuel cell that runs without added pressurization, humidification and/or heat, and therefore without any of the extraneous hardware of prior fuel cells.